When ducts are designed to carry such gases as flue and exhaust gases and gases resulting from various processes, expansion joints are essential components in order to accommodate thermal stresses.
One widely used type of expansion joint has an outer fabric cover and an inner shield. Such expansion joints have not been satisfactory when substantial lateral movement or combined lateral and axial movements must be tolerated, in preventing direct exposure of the fabric to the hot gases and the entrapment of dusts in the joints.
The duct gases may and often do carry abrasive particles and they may be of a corrosive nature. Such ducts are typically large in cross section and a representative example is the duct work required for a power plant utilizing a boiler fired with pulverized coal.
Present requirements for the use of gas cleaning equipment at power plants has resulted in complex duct work making necessary the use of expansion joints on the upstream and downstream sides of each component.
A typical power plant provided with a boiler utilizing pulverized coal as fuel has between the economizer outlet and the stack a precipitator, an air preheater, fan, and gas scrubber, and gas reheater. The gas temperature that is initially high is below the dew point and highly corrosive downstream of the scrubber so that expansion joints must be capable of meeting various requirements to avoid failures due to excessive thermal stresses, abrasion and corrosion, temperature and a high incident of dust.